It is known in the art of position determining devices sometimes referred to as digitizers to provide a tablet with a grid comprising a series of parallel equidistant magnetostrictive wires which are connected to a common transverse electrical conductor for exciting the magnetostrictive wire with a pulse of electrical current. In such devices the parallel magnetostrictive wires are arrayed parallel to the direction of the distance to be measured between the pointer and the reference origin. An electrical pulse is applied to the electrical conductor to excite the magnetostrictive wires and induce traveling strain waves along the wire. The apparatus for generating the electrical pulse is also used to start a timer. The pointer which may be a cursor or a stylus is provided with an inductive coil which generates a secondary pulse in response to the arrival of a strain wave on one or more of the adjacent magnetostrictive wires and the secondary pulse is used to stop the timer. The timer thus provides an indication of the time of travel of the strain wave from the origin to the pointer which is proportional to the distance of the pointer from the origin.
Magnetostrictive digitizers of the former type suffer from inaccuracies due to nonlinearities in the individual magnetostrictive wires. As a result of nonuniformities along the magnetostrictive wires as well as among the wires, it may take a magnetostrictive strain wave more or less time to traverse a length of one wire than to traverse the same length of a different portion of the same wire or of a different wire.
In order to overcome the problem of variations within and among magnetostrictive wires, digitizers have been constructed wherein a ratio is taken between the time of travel of the magnetostrictive strain wave between the pointer and reference point and the time of travel of the magnetostrictive strain wave along the entire length of each magnetostrictive wire and multiplied by a constant. Although the latter approach has proven to be more accurate than the former arrangement, it has not fully overcome the problems inherent in magnetostrictive digitizers which employ numerous magnetostrictive wires having variations within and among them. Such digitizers require large quantities of magnetostrictive wire to traverse the dimensions of the tablet usually in two coordinate directions, numerous times. The more magnetostrictive wire that is used, the greater the number of nonlinearities and variations and the more difficult it becomes to compensate for them. Increased cost and difficulty of manufacture are additional problems attending increases in the amount of magnetostrictive wire used. Additionally, in prior art digitizers, the surface of the tablet must be supported above the magnetostrictive wires if the surface is to receive a paper which is to be written upon, as pressure transmitted through the surface to the magnetostrictive wires is apt to interfere with strain wave propagation and, hence, coordinate measurement.
Another type of digitizer employs an electrical delay line parallel to each coordinate direction about which there are looped the wires of a matrix. This type of digitizer is disclosed in U.S. Pat. No. 3,648,277 to Whetstone et al for a Magnetic Graphical Data Device. Pulses are induced in the delay line and the delay time between the induction of a pulse in the delay line and the reproduction of the pulse at the end of the delay line is used to derive the corresponding coordinate of a signal generating stylus. However, since delay lines having a length on the order of digitizer tablet dimensions generally include delay circuitry at discrete points therealong with signals traveling almost instantaneously between the delay points, resolution of coordinate measurement is severely limited by the characteristics of the delay line. The previous type of digitizer is therefore unsuitable for use in applications requiring high precision coordinate measurement on tablet surfaces of relatively small dimension.